Conventional induction motors maintain the full sine wave of voltage across the stator winding regardless of the load on the motor. In those cases where the load varies within wide limits, e.g., when the motor is used for hoisting operations or in machine tools such as lathes, drill presses, etc., most of the time the motor is not expending its full rated load. In such cases, the iron losses in the stator are substantially the same when the motor is operating below full rated load as in the case when the motor is operating at full rated load; and due to the low power factor in such cases, the stator current is high and the copper losses are also substantial.
When the motor is operating below its full rated load, a fraction of the sine wave of voltage would satisfy the actual load requirement imposed on the motor, and this would in turn result in considerably lower iron and copper losses and less heating of the stator. The resultant lower operating temperature would further reduce the copper losses in the motor due to lowered ohmic resistance. These factors combine to effect a significant reduction in the energy which is consumed by the induction motor, with a consequent conservation in available energy resources and a reduction in motor operating costs.
In prior copending Parker U.S. application Ser. No. 839,945, filed Oct. 6, 1977, now abandoned in favor of continuation application Ser. No. 917,698 filed June 21, 1978, for "Energy Economizer for Induction Motors" a system is described which accomplishes the foregoing purposes by reducing energy losses when less than full rated load is applied to the induction motor. In the system described in said prior application, the induction motor is a standard AC induction motor having a stator winding which is energized from a sine wave power source through wave modifier means operative to vary the portion of each cycle of said sine wave which is coupled from the source to the stator winding, under the control of a control signal which is provided by load detecting means taking the form of a gear-tooth generator that is mechanically coupled to the shaft of the induction motor and which operates to provide a control signal that varies with variations in the load on the motor. The signal which is generated by said separate AC generator has a frequency which varies with variations in the load on the motor, and said frequency variations are converted to amplitude variations by supplying the signal to a frequency discriminator. The output signal from the frequency discriminator then controls wave modifier means comprising a normally inoperative pulse generator, such as a multivibrator, that is rendered operative to produce a train of pulses during a portion of each cycle from the sine wave energization source, the time duration during which the pulse generator is operative to produce output pulses being dependent upon the load on the motor. The pulse train, when generated, controls the closure of a full wave solid state power switch, such as a Triac assembly, disposed between the sine wave power source and the motor stator winding, so that the portion of each sine wave which is coupled to the motor stator winding, and accordingly the field density of the stator, varies with variations in the load on the motor.
The present invention is intended to accomplish results similar to those described above, without using a separate gear-tooth generator and an associated frequency discriminator.